This invention pertains generally to radio frequency communication systems, and more particularly to transponders for use in such systems.
As is known in the art, it is sometimes desired to use, in radio frequency communication systems, transponders to amplify and then retransmit received radio frequency signals. Such transponders generally include a radio frequency amplifier, such as a traveling wave tube (TWT), to provide the desired amplification. In order to retransmit a received radio frequency signal it is generally desirable for the transponder to perform linearly over its bandwidth. For example, if the radio frequency amplifier is operated in a nonlinear region near saturation, a distorted version of the received signal would be retransmitted. One type of distortion which results from such operation is intermodulation distortion. For example, if a transponder is used in a radio communication system to relay, simultaneously, at least two signals, each having a different frequency, such signals will intermodulate with each other when the amplifier is operated in its nonlinear region near saturation, thereby producing "cross talk" between the signals. In another application, as in a missile application, a missile carries the transponder to relay target reflected radio frequency energy to a ground station. In addition to target reflected radio frequency energy, however, reflections from clutter are also received by the transponder. Therefore, by operating the radio frequency amplifier in its nonlinear region near saturation intermodulation between the clutter reflections may produce frequency components at the frequency associated target reflections. The "signal-to-noise" ratio of the retransmitted radio frequency signal is therefore reduced relative to the signal-to-noise ratio of the received radio frequency signal.
In radio communication systems wherein a satellite includes a transponder, or in the above described missile application, it is highly desirable that the transponder be compact, lightweight and require minimum operating power. Therefore, in view of the foregoing, it follows that the linearity of a radio frequency amplifier used therein be optimized over the bandwidth of the transponder. The radio frequency amplifier used in such applications must, generally, then be operated about 10 db below the level where saturation of such amplifier begins in order to insure the requisite operating linearity. Generally, however, radio frequency amplifiers operate with 10-25 percent efficiency in the nonlinear region near saturation and consequently by requiring such amplifier to operate 10 db below saturation the efficiency of such amplifiers reduces to about an efficiency of 1 to 3 percent.